249 research outputs found
On the penetration of nucleons through heavy nuclei
The rate of loss of energy of a nucleon traversing a nucleus and the mechanism of star production are investigated on the basis of the meson type of interaction e-kr/xr. The theoretical predictions are compared with the available empirical data
Quality assurance in transnational higher education : a case study of the tropEd network
Transnational or cross-border higher education has rapidly expanded since the 1980s. Together with that expansion issues on quality assurance came to the forefront. This article aims to identify key issues regarding quality assurance of transnational higher education and discusses the quality assurance of the tropEd Network for International Health in Higher Education in relation to these key issues.; Literature review and review of documents.; From the literature the following key issues regarding transnational quality assurance were identified and explored: comparability of quality assurance frameworks, true collaboration versus erosion of national education sovereignty, accreditation agencies and transparency. The tropEd network developed a transnational quality assurance framework for the network. The network accredits modules through a rigorous process which has been accepted by major stakeholders. This process was a participatory learning process and at the same time the process worked positive for the relations between the institutions.; The development of the quality assurance framework and the process provides a potential example for others
Spontaneous Charging and Crystallization of Water Droplets in Oil
We study the spontaneous charging and the crystallization of spherical
micron-sized water-droplets dispersed in oil by numerically solving, within a
Poisson-Boltzmann theory in the geometry of a spherical cell, for the density
profiles of the cations and anions in the system. We take into account
screening, ionic Born self-energy differences between oil and water, and
partitioning of ions over the two media. We find that the surface charge
density of the droplet as induced by the ion partitioning is significantly
affected by the droplet curvature and by the finite density of the droplets. We
also find that the salt concentration and the dielectric constant regime in
which crystallization of the water droplets is predicted is enhanced
substantially compared to results based on the planar oil-water interface,
thereby improving quantitative agreement with recent experiments.Comment: 10 pages, 7 figures, submitted for publicatio
Ion association in low-polarity solvents: comparisons between theory, simulation, and experiment
The association of ions in electrolyte solutions at very low concentration
and low temperature is studied using computer simulations and quasi-chemical
ion-pairing theory. The specific case of the restricted primitive model
(charged hard spheres) is considered. Specialised simulation techniques are
employed that lead to efficient sampling of the arrangements and distributions
of clusters and free ions, even at conditions corresponding to nanomolar
solutions of simple salts in solvents with dielectric constants in the range
5-10, as used in recent experimental work on charged-colloid sus- pensions. A
direct comparison is effected between theory and simulation using a variety of
clustering criteria and theoretical approximations. It is shown that
conventional distance-based cluster criteria can give erroneous results. A
reliable set of theoretical and simulation estimators for the degree of
association is proposed. The ion-pairing theory is then compared to
experimental results for salt solutions in low-polarity solvents. The agreement
is excellent, and on this basis some calculations are made for the screening
lengths which will figure in the treatment of colloid-colloid interactions in
such solutions. The accord with available experimental results is complete
Liquid-liquid interfacial tension of electrolyte solutions
It is theoretically shown that the excess liquid-liquid interfacial tension
between two electrolyte solutions as a function of the ionic strength I behaves
asymptotically as O(- I^0.5) for small I and as O(+- I) for large I. The former
regime is dominated by the electrostatic potential due to an unequal
partitioning of ions between the two liquids whereas the latter regime is
related to a finite interfacial thickness. The crossover between the two
asymptotic regimes depends sensitively on material parameters suggesting that,
depending on the actual system under investigation, the experimentally
accessible range of ionic strengths can correspond to either the small or the
large ionic strength regime. In the limiting case of a liquid-gas surface where
ion partitioning is absent, the image chage interaction can dominate the
surface tension for small ionic strength I such that an Onsager-Samaras
limiting law O(- I ln(I)) is expected. The proposed picture is consistent with
more elaborate models and published measurements.Comment: Accepted for publication in Physical Review Letter
Stability of additive-free water-in-oil emulsions
We calculate ion distributions near a planar oil-water interface within
non-linear Poisson-Boltzmann theory, taking into account the Born self-energy
of the ions in the two media. For unequal self-energies of cations and anions,
a spontaneous charge separation is found such that the water and oil phase
become oppositely charged, in slabs with a typical thickness of the Debye
screening length in the two media. From the analytical solutions, the
corresponding interfacial charge density and the contribution to the
interfacial tension is derived, together with an estimate for the
Yukawa-potential between two spherical water droplets in oil. The parameter
regime is explored where the plasma coupling parameter exceeds the
crystallization threshold, i.e. where the droplets are expected to form
crystalline structures due to a strong Yukawa repulsion, as recently observed
experimentally. Extensions of the theory that we discuss briefly include
numerical calculations on spherical water droplets in oil, and analytical
calculations of the linear PB-equation for a finite oil-water interfacial
width.Comment: 9 pages, 4 figures, accepted by JPCM for proceedings of LMC
Optimality in superselective surface binding by multivalent DNA nanostars
Weak multivalent interactions govern a large variety of biological processes
like cell-cell adhesion and virus-host interactions. These systems distinguish
sharply between surfaces based on receptor density, known as superselectivity.
Earlier experimental and theoretical work provided insights into the control of
selectivity: Weak interactions and a high number of ligands facilitate
superselectivity. Present experimental studies typically involve tens or
hundreds of interactions, resulting in a high entropic contribution leading to
high selectivities. However, if, and if so how, systems with few ligands, such
as multi-domain proteins and virus binding to a membrane, show superselective
behavior is an open question. Here, we address this question with a multivalent
experimental model system based on star shaped branched DNA nanostructures (DNA
nanostars) with each branch featuring a single stranded overhang that binds to
complementary receptors on a target surface. Each DNA nanostar possesses a
fluorophore, to directly visualize DNA nanostar surface adsorption by total
internal reflection fluorescence microscopy (TIRFM). We observe that DNA
nanostars can bind superselectively to surfaces and bind optimally at a valency
of three. We quantitatively explain this optimum by extending the current
theory with interactions between DNA nanostar binding sites (ligands). Our
results add to the understanding of multivalent interactions, by identifying
microscopic mechanisms that lead to optimal selectivity, and providing
quantitative values for the relevant parameters. These findings inspire
additional design rules which improve future work on selective targeting in
directed drug delivery.Comment: 14 pages, 4 figure
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